The present invention relates to the field of microcontrollers and similar electronic devices.
In the arena of complex integrated circuits, there are sometimes different portions of the circuit that require voltage references for proper functioning. A voltage reference provides a precise output voltage, one that is much more accurate than the power supply voltage, which can vary as much as +/xe2x88x9210%. Its output voltage is compared to other voltages in a system and, usually, adjustments are made to those other voltages based on the reference difference. References are similar to regulators in how they function, but they are used much differently. While regulators are used to deliver power to a load, references are normally used with a small, stable load (if any) to preserve their precision. Only a few of the existing reference designs have the capability to deliver a load greater than a few milliamps while maintaining a precision output voltage.
A reference is not used to supply power, but to provide a system with an accurate analog voltage for comparison purposes, even when the system experiences a large temperature change. The band-gap reference circuit has long been used in integrated circuits for that purpose.
Some early band-gap reference circuits used conventional junction-isolated bipolar-IC technology to make relatively stable low-voltage references. This type of reference became popular as a stable voltage reference for low-voltage circuits, such as in 5-volt data acquisition systems where zener diodes were not suitable. It is important to note that band-gap circuits also are now widely used in digital ICs to provide a local bias that is less adversely affected by ambient noises, transients or temperatures.
A band-gap reference takes advantage of the electrochemical properties of a material. In a semiconductor, the amount of energy which allows the material to become conductive, i.e. move current in the presence of a voltage, is known as the band gap energy. The band gap energy is different for a variety of materials. However, silicon, the foundation material for a preponderance of integrated circuits, has a predictable band-gap energy that changes little with temperature over most of the temperature range of normal integrated circuit operations.
The nominal temperature coefficient of a silicon diode is xe2x88x922 mV/xc2x0 C. However, the temperature coefficient is inversely proportional to the current density in the diode. By manipulating the current densities through two diodes and taking the difference in forward bias voltages, a circuit that provides a voltage with a well-defined positive temperature coefficient can be created. This voltage is then added to the forward bias voltage of a 3rd diode. The positive temperature coefficient of the voltage cancels the negative temperature coefficient of the 3rd diode and one is left with a circuit with a nearly zero temperature coefficient. In practice, the diodes are generally the base-emitter junctions of integrated circuit transistors.
Modern band-gap circuits provide a gain adjustment, made in process, to compensate for process errors. Trimming can also, currently, be accomplished post-process by laser trimming of the integrated circuit, an expensive addition to the manufacturing process. Such trimming might have to be done in high precision ICs to accommodate very minor variations in the absorption characteristic of the silicon encountered in manufacture., No band-gap circuit, however, currently offers trimmability that can be changed after an initial trim. Nor does any current band-gap circuit offer trimmability while in operation.
What is needed, then, is a band-gap reference circuit that can account and compensate for process state differences and process error differences without requiring laser trimming after manufacture. A further need exists for such accounting and compensation to be changed when conditions change. Furthermore, the band-gap reference circuit must be trimmable while in operation.
Presented herein is a band-gap reference circuit that accounts and compensates for process state differences and process error differences without requiring laser trimming post manufacture. The present invention further discloses such a band-gap reference circuit that allows accounting and compensation to be changed when conditions change. It also provides for digital trimmability while in operation.
The present invention relates to a band-gap reference circuit. The circuit comprises a plurality of diodes connected in series in one or more chains, a current source to flow current through the diode chains, and a selection of shunt current sources. The shunt current sources are connected in parallel with the main current sources and each, or any, can be selected in order to add current to the diode chain. In this manner, current flow through the diode chain is adjusted in order to provide a trimmable band-gap reference voltage. By adjusting the current flow, the high precision reference voltage circuit can provide a very accurate reference value for variations in process state, process error and temperature.
Embodiments of the present invention include a band-gap reference circuit in an electronic device which comprises a band-gap reference and a plurality of selectable shunt diodes wherein the shunt diodes can be selected for activation by logic.